lol, so you did. yeah I just wasn't having to good a morning yesterday, nor today for that matter (save me caffine!!!!!) and just misread what you wrote.

I know there are those out there working ona concept of ramjet that can transition to a scram jet. Of course it would be helpful to get a scramjet just to work well enough in the first place...lolI know the USAF has a launch coming up sometime early this year i believe of a Boeing X-51 (http://en.wikipedia.org/wiki/Boeing_X-51) supposedly will operate for roughly 5 minutes at close to mach 6 and while operating will burn JP-7, the same fuel as the SR-71.

would be nice, but probably unlikely as its not related to "Earth Science" and "Global Warming Research" (no... I'm not bitter about the administration's direction for NASA, not at all )

in the end, a combined cycle engine of some type should be the next direction for cheap LEO, IMHO. whether it be for Crew to LEO, or heavy lift boosters,it seems like the way to go to increase the launch efficiency. now I know that the type of engine that would be used for the C2LEO missions would certainly be far different from the Cargo to LEO, but the potential is there.

personally I would love to play with a couple of GE J58 engines and try to build a small combined cycle system out of them. )turbojet/fan-assist ramjet/aug-rocket/rocket) cycle isn't too hard conceptually....lol. but there is where the math gets tricky and the fun begins.

the only problems with these kinds of hobbies, is where do you get the money to do them? lol

I dont see why you couldn't build a small scale combined cycle engine. Getting a small jet engine that could power a small, unmanned craft is hardly impossible. In fact i've seen people build primitive ones using turbochargers out of a car. (granted I doubt they worked very well...lol)

dont know how hard itis to get a hold of those engines anymore.... but I dont see why something couldnt be done with them. of course, now that i think of it, the issue would be getting the craft to a speed where the ramjet portion would work properly. without digging into some books and doing some math, there isn't a way to know short of actually building it (and even then, mucho math involved...lol).

I don't think jet engines for rocket launchers make any sense at all. The best of them have a thrust/weight of less than 20 rather than 80-120 for rocket engines. Then juts cannot work beyond about mach 3, about 1/10 of orbit launch requirement (or 1/100 the energy). So why not use the little extra propellant mass (mostly the LOX) and use rocket from the ground up? The propellant is a lot cheaper than the extra engine.

They make great sense because the specific impulse of turbojet/turbofans is 10+ times higher. Modern air breathing engines use the atmosphere as propulsive/inertial mass with only a small fraction of intake air being burned with fuel. The intake air also cools the engine very simply and reliably. So the engines run cooler.

A rocket must carry all of its propulsive energy and inertial mass and deal with higher temperatures, higher pressures, a much more complex cooling system and possibly ultra high RPM turbopumps to save weight.

So if the turbojet/turbofan gets you to an altitude above 90% of the atmosphere with 1/10 the fuel, higher reliability and lower engine costs, that's a huge advantage.